The present invention relates generally to amplifier circuits and, in particular, to providing negative feedback to an amplifier circuit that includes an output lowpass filter.
Class D power amplifiers are well-known amplifiers for use in audio applications. Such amplifiers commonly use a lowpass filter between an output switching power stage and the load (e.g., speaker) to substantially eliminate a majority of the high-frequency components of the switched power signal, thereby leaving the desired audio signal. Class D amplifiers also commonly use negative feedback to reduce the distortion and output impedance of the amplifier.
Negative feedback in a class D amplifier is commonly accomplished by feeding back either the input signal to the output lowpass filter or the output signal of the output lowpass filter. A conventional class D amplifier 100 in which the input signal to the output lowpass filter 107 is fed back is depicted in electrical block diagram form in FIG. 1. The amplifier 100 includes an error amplifier 101, a pulse width modulator (PWM) 103, and an output amplification stage 105. The amplifier 100 drives a load 109, such as an audio speaker. A single-pole lowpass filter 111 may be optionally inserted in the feedback path to remove some of the high switching frequency components of the output signal of the output stage 105 from the feedback signal 115. The feedback approach depicted in FIG. 1 avoids feedback loop instability problems that may result when the feedback signal 115 is taken from the output of the output lowpass filter 107, such instabilities being due to the phase shift and amplitude peaking of the filter 107. The drawback of the amplifier circuit 100 of FIG. 1 is that the output lowpass filter 107 is outside of the feedback loop, so any low frequency, non-ideal behavior of the filter 107 is not corrected by the feedback and thus degrades amplifier performance.
A conventional class D amplifier 200 in which the output signal of the output lowpass filter 107 is fed back is depicted in electrical block diagram form in FIG. 2. In this circuit 200, the feedback is taken from the actual output of the amplifier 200. With the approach of FIG. 2, the output filter 107 is within the feedback loop. Thus, the output filter""s non-ideal behavior can be corrected by the feedback loop in order to reduce the performance degradation normally introduced by the filter 107. In practice, however, the approach depicted in FIG. 2 is difficult to apply and requires compromises which degrade other aspects of amplifier circuit performance.
Because the output lowpass filter 107 is within the feedback loop in the circuit 200 of FIG. 2, the filter""s phase shift and amplitude response must be accounted for when compensating the feedback loop to ensure amplifier stability. When used, the output lowpass filter 107 is typically implemented to have a transfer function with two or more poles. More than one pole is typically necessary to provide acceptable attenuation of the high-frequency components present in the output stage""s output signal, without significant attenuation of desired audio frequencies. Thus, the output filter 107 adds at least ninety degrees (90xc2x0), and typically adds one hundred eighty degrees (180xc2x0) or more, of phase shift at some particular frequency to the open-loop phase shift of the amplifier circuit 200. Such substantial open-loop phase shift makes feedback loop compensation difficult because general engineering practice restricts the total open-loop phase shift to a maximum of one hundred thirty-five degrees (135xc2x0) at the unity loop-gain crossover frequency.
One approach that is conventionally used to compensate for the effects of the output lowpass filter 107 when the filter 107 is within the feedback loop is the insertion of a phase compensation network 201 into the feedback path as shown in FIG. 2. The phase compensation network 201 cancels at least some of the phase shift introduced by the output filter 107. However, the phase compensation provided by the phase compensation network 201 only works over a narrow range of frequencies and requires that the phase and amplitude responses of the output filter 107 be known and vary within known limits under all operating conditions.
Moreover, the phase and amplitude responses of the output filter 107 are dependent upon the impedance of the load 109. Consequently, when the impedance of the load 109 is not under the control of the amplifier circuit designer, such as when the amplifier circuit 200 is sold as a stand-alone unit, a priori knowledge of the phase and amplitude responses of the output filter 107 is at best difficult, if not impossible, to estimate.
Furthermore, use of a phase compensation network 201 is typically only effective for compensating phase shifts introduced by output lowpass filters 107 having transfer functions with one or two poles. Thus, the use of a feedback phase compensation network 201 is inadequate for compensating desirable higher-order filters that provide greater attenuation of the high-frequency components of the output stage""s output signal.
As an alternative to utilizing a feedback phase compensation network 201, the open-loop bandwidth of the amplifier circuit 200, excluding the output lowpass filter 107, can be reduced such that the unity loop-gain crossover frequency falls below the frequency at which the output lowpass filter 107 adds enough phase shift to cause loop instability. However, such a bandwidth limitation severely restricts the operating frequency range of the amplifier circuit 200 and reduces the available loop gain at operating frequencies, thereby increasing distortion and output impedance as compared to using the feedback phase compensation network 201.
Therefore, a need exists for an amplifier circuit and method of providing feedback thereto that effectively removes the output lowpass filter from the feedback loop at high frequencies, thereby eliminating the drawbacks associated with either the use of a feedback phase compensation network or limiting the open-loop bandwidth of the amplifier circuit, and includes the output lowpass filter in the feedback loop at low frequencies to enable the feedback loop to correct for the non-ideal behavior of the filter.